HUDSON-RIVER GROUP. 
255 
TRILOBITES OF THE HUDSON-RIVER GROUP. 
307. 1. TRINUCLEUS CONCENTRICUS. 
Pl. LXVII. Figs. 1 a-h. Also Pl. LXV. Fig. 4. 
The specimens from the slates of the Hudson-river group are more flattened than those 
from the Trenton limestone. In a large number of specimens examined, there are four 
rows of punctures in front of the buckler, while a few have but three rows ; on the other 
hand, those with four rows are rare in the Trenton limestone, while those with three rows 
are the prevailing form. This difference, however, cannot be regarded as essential, for in 
a few instances there are five rows, and very frequently five or six rows on the maxillary 
margin, near the posterior termination of the buckler. 
The specimens in the Hudson-river group rarely preserve the posterior spine of the 
glabella, but it is shown in fig. 1 d, and is never wanting except from accident. The long 
spines from the posterior angles of the buckler correspond in the specimens from the two 
rocks. 
The thorax consists of six free articulations, the lateral ones being bifurcate or grooved; 
the caudal shield has seven segments in each lateral lobe, and about fourteen in the central 
lobe. The crust is exceedingly thin, almost always separating; and when preserved in the 
shaly strata, rarely shows any surface markings. The animal had the power of contracting 
or folding itself, at least to a certain degree, though this does not appear to have been its 
habit. 
Fig. I a. A large individual, destitute of spines upon the posterior angles of the buckler and glabella. 
The border in front of the glabella has three distinct rows of punctures, four in front of 
the cheeks, and five on each side, with six or seven near the base. 
Fig. 1 b. The thorax and caudal shield enlarged, to show more clearly the character of the segments. 
Fig. 1 c. A smaller specimen, preserving the spines of the buckler. There are four rows of punctures 
in front, and six on each side of the buckler. 
Fig. 1 d. The buckler, preserving the posterior spine of the glabella. 
These specimens are from the soft shales in the upper part of the Hudson-river group, at 
Loraine. 
Fig. 1 e. Fragments of the cephalic border, showing a variable number of rows of punctures in 
front; one having five, with seven or eight at the posterior margin. 
Fig. 1 /. A portion of the marginal fillet, where the crust is partially removed, showing the little studs 
or points which fill these pores from below. 
